Research On The Orientation Distribution Of Fibers And Rheological Property In Fiber Suspensions Turbulent Flow

Fiber suspension flow discussed in this dissertation is a kind of two-phase flow which contains cylindrical rigid fibers in Newtonian fluid. Fiber suspension flow is an important part of multiphase flow and it is widely applied to composite materials production, textile industry, chemical industry, paper industry and many other areas. Nowadays, the researches on the flow of fiber suspensions in turbulence and the coupling effect of suspending fibers and turbulence are the key point and difficult point.To explore the orientation distribution of fibers and rheological property in turbulent flow of fiber suspension, the fiber suspensions in turbulence were theoretically analyzed and simulated. The equation of probability distribution function for mean fiber orientation was derived. The relationship between the correlation of tow fluctuating quantities and the gradient of mean orientation probability distribution function was built. The function of average probability distribution was solved numerically in turbulent channel flow of fiber suspensions. The results show that the fibers tend to align with the flow direction, but this trend is not as strong as that in the laminar flow. The orientation distributions retain unchanged along the lateral direction. The fibers align along the flow direction more obviously with increasing the fiber aspect ratio. The fiber orientation distribution is not dependent on the pressure gradient of the flow. The shear stress and the first normal stress difference decrease from wall to the centerline and approaches to zero at the centerline. The shear stress does not vary as the fiber aspect ratio and faction of volume, while the absolute value of the first normal stress difference increases with increasing the faction of volume and decreasing the fiber aspect ratio.In the dilute and semi-dilute fiber suspensions turbulent channel flow, the equation of probability distribution function for mean fiber orientation was solved and then the equation of Reynolds averaged Navier-Stokes with the term of additional stress resulted from fibers was also solved simultaneously to explore the characteristics of fiber suspension flow. Further the terms of effect resulted from fibers in the equation of turbulence kinetic energy k and equation of turbulence energy rate of dissipationεwere obtained respectively. The numerical research show that the results which employ the equations k andεwith the terms of effect by fibers were more accurate than that employ the equations k andεwithout the terms of effect by fibers.In the turbulent pipe flows of dilute and semi-dilute fiber suspension, the second-and fourth-order orientation tensor of the fiber in turbulent suspension flow were obtained, and the equation of Reynolds averaged Navier-Stokes with the term of additional stress resulted from fibers were solved to explore the characteristics of turbulent fiber suspension.The effect of Reynolds number, fiber concentration and fiber aspect-ratio on the mean velocity were presented and analyzed in the research on the. fibers suspension turbulent flow in channel and pipe. It was found that the velocity profile of fiber suspension becomes sharper with decreasing the flow rate and/or increasing the fiber concentration, which agrees with the corresponding experimental ones. The velocity profile becomes steeper as fiber aspect-ratio increases. Fibers at this range of concentration and scales play a role in the increase of viscosity, and that with the increase of fiber concentration and fiber aspect-ratio the effect of that enhances. The effect of Re for suspension flow was similar to the role of single phase turbulent flow. In the dilute and semi-dilute fiber suspensions flow when the volume concentration of fiber less than 1%, the turbulence effect on the fiber suspension flow plays an important role to the fiber suspension. The turbulent kinetic energy will increase with increasing fiber concentration and fiber aspect-ratio. The turbulent dissipation rate will also increase with increasing fiber concentration and fiber aspect-ratio.Finally, the prediction formula of velocity profile involving the effect of Reynolds number, fiber concentration and aspect-ratio for the turbulent pipe flow and channel flow of fiber suspension were derived.